Antireflection Performance of the Conical Microstructures of Germanium Substrate in Long-Wavelength Infrared
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摘要: 锗是重要的红外光学材料，为减小锗表面的菲涅耳反射损耗，提高光利用率，研究了锗基底圆锥形微结构的减反射性能。基于时域有限差分法（Finite Difference Time Domain），并采用单因素法研究了微结构的占空比、周期、高度等结构参数与入射角在8~12 μm长波红外波段对反射率的影响，确定了微结构在低反射情况下较优的结构参数组合，其在整个波段范围内的平均反射率低于1%，远低于平板锗结构的35.47%，在9~11 μm的波段范围内反射率低于0.5%，且光波在40°范围内入射时，圆锥形微结构的平均反射率仍然较低。将优化的圆锥形微结构与平板结构进行了对比，从等效折射率、反射场分布和能量吸收分布3方面进一步证实了圆锥形微结构在整个波段范围内优异的减反射性能。Abstract: Germanium is an important infrared optical material. To reduce Fresnel reflection loss on the germanium surface and improve the light utilization rate, the anti-reflection performance of the conical microstructure on a germanium substrate was studied. Based on the finite difference time domain (FDTD) method and the single factor method, the effects of the microstructure parameters, such as duty ratio, period, height, and the angle of incidence on reflectivity are discussed for the 8 μm to 12 μm long-wavelength infrared band. The structural parameters of the microstructure at low reflection was determined. Its average reflectivity over the entire wavelength range is less than 1%, which is much lower than the 35.47% reflectivity of the slab germanium structure, and the reflectivity in the wavelength range of 9 μm to 11 μm is less than 0.5%. The average reflectivity of the conical microstructure remained low when light was incident at 40°. By comparing the optimized conical microstructure with the slab structure, the excellent antireflection performance of the conical microstructure over the entire wavelength range was further confirmed based on the equivalent refractive index, reflected electric field intensity distribution, and absorption per unit volume.
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图 1 FDTD仿真模型及结构参数示意图。(a) 圆锥形微结构模型的三维示意图；(b) 平板型结构模型的三维示意图；(c) 圆锥形微结构模拟设置及结构参数；(d) 圆锥形微结构实际模拟单元
Figure 1. Schematics of FDTD simulation model and structural parameters. (a) Three-dimensional schematic of the conical microstructure model; (b) Three-dimensional schematic of the slab structure model; (c) The Conical Microstructural Simulation Setup and Structural Parameters; (d) The Conical Microstructure actual simulation unit cell
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图 2 单晶锗的折射率与波长的关系曲线
Figure 2. The relationship between the refractive index and wavelength of single crystal germanium
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图 3 占空比对反射率的影响。(a) 不同占空比与反射率的关系；(b) 占空比为0.8~1时与反射率的关系
Figure 3. The effect of Duty ratio on reflectivity. (a) The relationship between different fill factor and reflectivity; (b) The reflectivity when the fill factor is 0.8-1
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图 4 周期对反射率的影响。(a) 不同周期与反射率的关系；(b) 周期为2.6~3.0 μm时与反射率的关系
Figure 4. The effect of period on reflectivity. (a) The relationship between different periods and reflectivities; (b) The reflectivity when the period is 2.6-3.0 μm
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图 5 圆锥形微结构高度对反射率的影响。(a) 不同高度与反射率的关系；(b) 高度为3.6~4.0 μm时与反射率的关系
Figure 5. The effect of conical microstructure height on reflectance. (a) The relationship between different heights and reflectivities; (b) The reflectivity when the height is 3.6-4.0 μm
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图 6 平板型与圆锥形微结构的反射曲线
Figure 6. Reflection curves of the slab structure and the conical microstructure
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图 7 0°~70°入射时圆锥形微结构平均反射率曲线
Figure 7. Average reflectance curve of the conical microstructure when light incidence is in the range of 0°-70°
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图 8 平板型与圆锥形微结构的等效折射率
Figure 8. Equivalent refractive indexes of the slab structure and the conical microstructure
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图 9 两种结构在Y＝0截面上9.6 μm、11.2 μm、12 μm波长下的反射场分布
Figure 9. The reflected electric field intensity distribution of the two structures at 9.6 μm, 11.2 μm and 12 μm on the Y＝0 section
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